1. Field of the Invention
A first aspect of the present invention relates to an optical pick-up system comprising:
a light source for emitting light;
an objective lens for causing light emitted from said light source to converge onto an optical recording medium;
a beam splitter for separating light, reflected from said optical recording medium, from the bundle of rays emitted from said light source;
a detective lens for causing said light separated by means of said beam splitter to converge; and
a detective photosensitive element disposed at a position at which the light converged by means of said detective lens approximately converges.
A second aspect of the present invention relates to an optical pick-up system used for writing optical information onto/reading optical information from/erasing optical information from an optical information recording disc such as a so-called compact disc or magneto-optical disc.
A third aspect of the present invention relates to an optical pick-up system comprising:
a light source for emitting light;
an objective lens for converging the bundle of rays emitted from said light source on an optical recording medium;
a beam splitter for separating the light, reflected by said optical recording medium, from said bundle of rays emitted from said light source; and
a detective optical system for introducing the separated reflected light thereto as detective light, said detective optical system then forming various signals.
2. Related Art
Such an optical pick-up system is used for writing optical information onto/reading optical information from an optical recording medium in an optical writing/reading apparatus such as a magneto-optical disc apparatus, write-once type optical disc apparatus, CD-ROM apparatus, and audio CD apparatus. Various kinds of focusing-error detecting methods are used in such an optical pick-up system. Such a focusing-error detecting method is used for detecting a focusing error of a laser beam, the laser beam being used for writing optical information onto/reading optical information from (and erasing optical information from) the optical recording medium.
In particular in the above various methods, the knife-edge method (refer to Japanese Patent Publication No.62-18973) and the Faucault method, that is the so-called double-knife-edge method (refer to Japanese Laid-Open Patent Application NO.20126429) are effective to obtain high detective sensitivity but have relatively simple constructions.
However, such methods may exhibit certain disadvantages, as described below.
Normally, high accuracy is required in the positioning adjustment of the light-reception surface of the detective photosensitive element. This is because the spot diameter of the detective bundle of rays is considerably small on the detective photosensitive element which receives the detective bundle of rays.
The shape of the detective bundle of rays on the light-reception surface is determined based on the spatial relationship between the detective lens for converging the detective bundle of rays and the knife edge inserted in the detective bundle of rays.
Therefore, in the case where the knife edge method is applied, high precision is required in disposing the knife edge such that the proportion of the bundle of rays to be blocked by the knife edge (light-blocking ratio) and the angle of obliqueness of the knife edge are appropriately adjusted. On the other hand, in the case where the Faucault method is applied there, high accuracy is required in positioning adjustment of the roof prism for splitting the bundle of rays. Thus, in both cases, the positioning adjustment is a difficult or delicate procedure.
In an optical pick-up system, focusing control is performed for properly converging the bundle of rays emitted from the light source on the recording surface of the optical recording medium. Focusing control using the knife edge method is effective because the detection sensitivity thereof is high but has a simple construction as mentioned above. However, in the case where the knife edge method is used, a disadvantage may occur wherein the part of bundle of rays blocked by the knife edge is not used redundant. Thus, the light is not used efficiently due to this light loss.
In order to eliminate such a light loss, the following optical pick-up system may be used, as shown in FIGS. 1A and 1B. FIG. 1A shows a system construction for a case where an optical information recording medium 6 comprises a write once read many type optical disc. Here a write once read many type optical disc means an optical disc of a type such that data once written on the disc cannot be erased or overwritten.
A diverging bundle of rays emitted by a semiconductor laser 1 is made parallel bundle of rays by means of a collimating lens 2. Then, the resulting parallel bundle of rays is incident on a beam forming prism 31 constituting a composite prism 3. As a result, the flux cross section of the incident bundle of rays is beam-formed to be a desired shape (a circle or a near-circular ellipse) by being refracted and reflected therein. Then, after being thus beam-formed, the resulting bundle of rays is incident on an objective lens 4 via an optical semi-transparent surface 32 and a normal prism 33. Then, by the function of the objective lens 4, the incident bundle of rays is made to converge a light spot on the recording surface of the optical information recording medium 6. The objective lens 4 is integrated with a linearly seeking member 5 and runs along a path which intersects the rotational axis of and is parallel to the recording surface of the optical information recording medium 6.
The bundle of rays reflected by the recording surface is returned to the composite prism 3 via the objective lens and then reflected by the semi-transparent surface 32. Thus, the bundle of rays is separated from the light path that starts from the light source and ends at the recording surface. The thus separated bundle of rays is converted into a converging bundle of rays by means of a detective lens 7. Then, a part of the converted bundle of rays is incident on a photosensitive element 10 as a result of being reflected by the reflective prism 8. The remaining part of the bundle of rays which has not been reflected by the reflective prism converges on a photosensitive element 9.
The photosensitive element 9 comprises one which has two light-reception surfaces so that the first light-reception surface is located at the upper part and the second light-reception surface is located at the lower part in FIG. 1A. The respective light-reception surfaces then output the light-reception signals A and B. On the other hand, the photosensitive element 10 comprises one which has two light-reception surfaces so that the first light-reception surface is located at the front part and the second light-reception surface is located at the back part along a direction perpendicular to the plane of FIG. 1A. The respective light-reception surfaces of the photosensitive element 10 output the light-reception signals C and D.
A focusing-error signal is produced from the difference (A-B) and a tracking-error signal is produced from the difference (C-D). A data signal (reproduction or reading signal) is produced from the sum (A+B+C+D).
FIG. 1B shows a detective system in a case where the optical information recording medium 6 comprises a magneto-optical disc. The generation of the focusing-error signal is similar to that in the case of FIG. 1A. In the case where the optical information recording medium comprises the magneto-optical disc, the reflection bundle of rays obtained from the reflective prism 8 is further polarization-split by a polarization beam splitter 11. Then, the thus separated bundles of rays are incident on photosensitive elements 9 and 10, respectively. The photosensitive element 10 is the same as that described with reference to FIG. 1A and therefore outputs the light-reception signals C and D. The photosensitive element 11 comprises one having a single light-reception surface and outputs the output signal E.
The focusing-error signal and tracking-error signal are formed, similarly to the case of FIG. 1A, of the signal (A-B) and the signal (C-D), respectively. However, the data signal is formed of [(C+D)-E].
For the sake of clear indication of the signals A, B, C, D and E as output signals, these letters are enclosed in brackets in FIGS. 1A and 1B.
In the optical pick-up systems shown in FIGS. 1A and 1B, the focusing control is carried out using the knife-edge method. However, there occurs no light loss such as described because the reflective prism 8 acts as the knife edge member for blocking a part of the bundle of rays converging on the photosensitive element 9. By this construction, the bundle of rays which has been blocked for the photosensitive element 9 is incident on the photosensitive element 10.
However, there may occur other disadvantages in the optical pick-up systems shown in FIGS. 1A and 1B such as follows: The photosensitive elements 9 and 10 for generating the focusing-error signal and the tracking-error signal respectively are disposed at the separate positions as shown in FIG. 1A. As a result, the task of adjustment of the spatial relationship among the photosensitive elements 9, 10 and the reflective prism 8 may be troublesome.
Further, errors may occur in the tracking control due to aging of the optical pick-up system. This is because, for reasons such as vibration applied to the system, the disposed posture of the reflective prism, for example may be slightly altered. Even such slight posture variation of the reflective prism results in considerable deflection of the reflected light.
Further, another example of an optical pick-up system for detecting the focusing error using the knife-edge method will now be described with reference to FIG. 2A indicating essential elements of the optical system.
In FIG. 2A, the bundle of rays emitted from a semiconductor laser element 51 is converted into a parallel bundle of rays by means of a collimating lens 52. Then, the thus converted parallel bundle of rays is transmitted by a separating surface 53a of a beam splitter 53 and then is incident on an objective lens 54. The incident bundle of rays is then made to converge on a recording surface of an optical disc 55 by means of the objective lens 54.
On the other hand, the reflection light reflected by the recording surface of the optical disc 55 is reflected by the division surface 53a of the beam splitter 53. Then, the light thus reflected by the separating surface 53a is incident on a detective lens 56 so as to be made converge. Almost all of the bundle of rays which has been transmitted by the detective lens 56 (referred to as detection bundle of rays, hereinafter) is reflected by a knife edge prism 57. The reflected light is then received by a dual-surface photosensitive element 58. On the other hand, the remaining part of the detection light which has not been reflected by the knife edge prism 57 is received by a dual-surface photosensitive element 59.
There, the light-reception surface of the dual-surface photosensitive element 58 is divided into two parts so that the division line extends in the direction in which the recording tracks (not shown in the drawing) extend on the recording surface of the optical disc 55 as shown in FIG. 2B. On the other hand, the light-reception surface of the dual-surface photosensitive element 59 is divided into two parts so that the division line extends in the direction of the ridgeline 57a of the knife edge prism extends as shown in FIG. 2C.
In the optical system shown in FIG. 2A, the dual-surface photosensitive element 59 is disposed at the focal point A of the detective bundle of rays converged by the detective lens 56 in a condition where the laser beam is focused on the recording surface of the optical disc 55, as shown by solid lines in FIG. 3A. There, the knife edge 60 indicates the knife edge effect of the knife edge prism 57.
Therefore, in the focused state, the spot of the detective flux formed on the light-reception surface of the dual-surface photosensitive element 59 is made to be one such as the spot SPa indicated in FIG. 3B by a solid line. In this state, the two areas, of the spot SPa, formed on the respective two light-reception surfaces of the dual-surface photosensitive element 9 are equal.
On the other hand, as shown by broken lines in FIG. 3A, there may be a case where the laser beam is slightly defocused on the recording surface of the optical disc 55. In such a case, the spot of the detection bundle of rays (the focal point is at the point B) formed on the light-reception surface of the dual-surface photosensitive element 59 is made, through the detective lens 56, to be one such as the spot SPb shown by a broken line in FIG. 3B. Therefore, in this state, the two areas, of the spot SPa, formed on the respective two light-reception surfaces of the dual-surface photosensitive element 9 are such that one is larger than the other. Such area biassing depends on the relevant defocusing direction.
Then, if the laser beam is further defocused, the spot of the detection bundle of rays, converging on the light-reflection surface of the dual-surface photosensitive element 59 due to the detective lens 56, is made to be one such as the spot SPc shown by a chain double dashed line in FIG. 3B.
Thus, it is possible to detect a focusing error of the objective lens 54 for the recording surface of the optical disc 55, based on the difference between the light-reception signals obtained from the respective two light-reception surfaces of the dual-surface photosensitive element 59. The knife-edge method such as described above may be widely used as a focusing-error detecting method having high sensitivity but a simple construction.
In the system shown in FIG. 3A, it is desired to reduce the diameter of the laser spot to be formed on a recording surface of the optical disc 55 so that the data recording density for the optical disc 55 may be improved. In order to reduce the diameter of the laser spot, it may be required to use a lens having a so-called high numerical aperture (abbreviated to be NA, hereinafter) property as the objective lens 55, such that the NA is more than 0.5.
However, if such a high NA objective lens is used in an optical pick-up system using the knife-edge method, the sensitivity in the focusing-error detection becomes too high, the focus range becoming thus extremely narrow.
In order to eliminate such a problem, the following counter measures may be considered:
a) To increase the focal length of the objective lens;
b) to shorten the focal length of the detective lens; and
c) to increase the blocking ratio of the knife edge 10. However, the following problems occur for the above measures:
a) If the NA is to remain unchanged, the incident beam diameter must be enlarged and thus the system must be scaled up;
b) the spot formed on the light-reception surface of the dual-surface photosensitive element 59 is reduced and as a result positioning adjustment of the dual-surface photosensitive element 59 becomes difficult and also the system becomes vulnerable to variations in the positions due to aging; and
c) the quantity/intensity of the detection bundle of rays incident on the dual-surface photosensitive element 59 is reduced and as a result the system is liable to be adversely affected by flare light and/or noise.
Further, the track-crossing-noise in the focusing-error signal becomes remarkable if a high NA objective lens is used, which noise seldom occurs when a low NA objective lens is used. As a result, loss of focus may occur during a so-called direct seeking operation in which the number of recording tracks which have been crossed is detected using the wave form of the tracking-error signal.
Such track-crossing-noise occurs as follows:
First, the amount of light reflected from the disc surface (detection light amount) differs between the cases where the spot position is in a groove and on a land (non-groove). Further, the shape of the reflected wave front (the wavefront shape of the detection bundle of rays) also differs between the cases of groove and land. The larger the NA of the objective is made to be, the larger such variation in the light amount and variation in the wavefront shape become.
As a result, in the knife-edge method in which a part of the detection bundle of rays is received at the detective-lens light-converging point, the following phenomena occurs. The focus spot is made to be a very small diffraction spot and the spot position varies due to the intensity of the detection bundle of rays or the wavefront shape. As a result, quivering occurs in the focusing-error signal. As an example of this quivering, when the intensity of the bundle of rays increases, the spot SPa of FIG. 3B slightly moves. The shorter the focal length of the detective lens becomes, the greater this spot-position movement becomes.